this series introduces the frontend for the newly introduced PV Callsprocotol.

PV Calls is a paravirtualized protocol that allows the implementation ofa set of POSIX functions in a different domain. The PV Calls frontendsends POSIX function calls to the backend, which implements them andreturns a value to the frontend and acts on the function call.

This patch series only implements the frontend driver. It doesn'tattempt to redirect POSIX calls to it. The functions exported inpvcalls-front.h are meant to be used for that. A separate patch serieswill be sent to use them and hook them into the system.

Changes in v7:- define sock_mapping earlier- make sure that every patch in the series builds- call pvcalls_front_free_map(map->passive.accept_map) without the socket_lock- remove now unused bool lock parameter to pvcalls_front_free_map- XEN_PVCALLS_FRONTEND: default n and select XEN_XENBUS_FRONTEND